1. Field of the Invention
The present invention relates to a substrate structure of a liquid crystal display and a fabrication method thereof, and in particular to a substrate structure of a liquid crystal display and a fabrication method thereof that are capable of improving the light transmittance of the liquid crystal display.
2. Description of the Background Art
In general, liquid crystal display is for displaying pictures by adjusting the light transmittance of liquid crystal cells. Data signals according to picture information are supplied to the liquid crystal cells individually arranged in a matrix form.
The liquid crystal display includes a liquid crystal display panel having pixel unit liquid crystal cells in a matrix form and a driver IC (integrated circuit) for operating the liquid crystal cells. The liquid crystal display panel includes a color filter substrate and a TFT (thin film transistor) array substrate facing each other and a liquid crystal layer filled between them.
On the TFT array substrate of the liquid crystal display panel, parallel data lines for transmitting data signals from a data driver IC to the liquid crystal cells and parallel gate lines for transmitting scanning signals from a gate driver IC to the liquid crystal cells cross each other at about 90° angles. A liquid crystal cell is defined at each cross portion of the data line and the gate line.
The gate driver IC sequentially selects the liquid crystal cells arranged in matrix form by one line unit by applying scanning signals to the plural gate lines sequentially, and the selected one line of the liquid crystal cells receive data signals from the data driver IC.
In the meantime, the color filter substrate and the TFT array substrate facing each other include a common electrode and pixel electrodes in order to apply electric field to the liquid crystal layer. The pixel electrode is allocated to each liquid crystal cell formed on the TFT array substrate. On the other hand, the common electrode is formed onto the entire surface of the color filter substrate as one body. Accordingly, when a voltage is applied to the common electrode, by controlling a voltage applied to each of pixel electrodes, light transmittance of the liquid crystal cells can be individually controlled.
As described above, in order to control the voltage applied to the pixel electrode for each liquid crystal cell, a TFT used as a switching device is formed at each liquid crystal cell.
The construction parts of the liquid crystal display will be described in more detail with reference to accompanying drawings.
First, FIG. 1 is a plan view illustrating a unit pixel of a general liquid crystal display.
As depicted in FIG. 1, in the unit pixel of the general liquid crystal display, gate lines 4 are arranged in rows at regular intervals, and data lines 2 are arranged in columns at regular intervals. Accordingly, the gate lines 4 and the data lines 2 are arranged in the matrix format. Herein, each unit liquid crystal cell is defined at each cross portion of the data line 2 and the gate line 4 and includes a TFT and a pixel electrode 14.
For each TFT, a gate electrode 10 is formed by being extended from a certain portion of the gate line 4, a source electrode 8 is extended from the data line 2, and accordingly the gate electrode 10 overlaps with a certain portion of the source electrode 8. A drain electrode 12 is formed at a portion corresponding to the source electrode 8 on the basis of the gate electrode 10. The pixel electrode 14 electrically contacts the drain electrode 12 through a drain contact hole 16 formed through the drain electrode 12.
The TFT also includes a semiconductor layer (not shown) for forming a conductive channel between the source electrode 8 and the drain electrode 12 as a scanning signal is applied to the gate electrode 10 through the gate line 4.
As described above, because the TFT forms the conductive channel between the source electrode 8 and the drain electrode 12 by receiving the scanning signal from the gate line 4, a data signal supplied to the source electrode 8 through the data line 2 is transmitted to the drain electrode 12 through this conductive channel.
The pixel electrode 14 in electric contact with the drain electrode 12 through the drain contact hole 16 is made of transparent ITO (indium tin oxide) having high light transmittance ratio. Herein, the pixel electrode 14 generates electric field at the liquid crystal layer with a common transparent electrode (not shown) formed on the color filter substrate by the data signal supplied from the drain electrode 12.
As described above, when the electric field is applied to the liquid crystal layer, liquid crystal is rotated by dielectric anisotropy and transmits light generated from a back light unit to the color filter substrate through the pixel electrode 14, and a quantity of the transmitted light is adjusted by a voltage value of the data signal. Generally, the back light unit includes a light source and reflector for supplying light to the liquid crystal panel for displaying pictures, images, etc.
A storage electrode 20 in electric contact with the pixel electrode 14 through a storage contact hole 22 forms a storage capacitor 18 by being deposited on the gate line 4. A gate insulating layer (not shown) deposited in the process of forming of the TFT is inserted between the storage electrode 20 and the gate line 4.
The storage capacitor 18 charges a voltage value of a data signal for a turn-on period of the TFT in which the scanning signal is applied to the gate lines 4, and supplies the charged voltage to the pixel electrode 14 for a turn-off period of the TFT. Accordingly, the operation of the liquid crystal is maintained.
FIG. 2 is an exemplary view illustrating a section of a unit pixel cut along line I-I′ in FIG. 1. As shown in FIG. 2, the unit pixel includes a color filter substrate 60 and a TFT array substrate 50 facing each other; a spacer 70 for separating the TFT array substrate 50 and the color filter substrate 60 from each other; and a liquid crystal layer 80 filled into the space between the TFT array substrate 50 and the color filter substrate 60.
The fabrication process of the TFT of the liquid crystal display will be described in detail with reference to FIG. 2.
First, the gate electrode 10 is formed by depositing a metal such as Mo, Al or Cr on the TFT array substrate 50 using a sputtering method and by patterning it through a first mask. A gate insulating layer 30 is then formed by depositing an insulating substance such as SiNx, etc. on the TFT array substrate 50 having the gate electrode 10. An active layer 36 of the TFT is formed by sequentially depositing a semiconductor layer 32 made of amorphous silicon and an ohmic contact layer 34 made of n+ amorphous silicon doped with impurities having high density on the gate insulating layer 30 and patterning them through a second mask.
The source electrode 8 and the drain electrode 12 of the TFT are then formed by depositing metal substance on the gate insulating layer 30 and the ohmic contact layer 34 and patterning it through a third mask. Herein, the patterning is performed so as to make the source electrode 8 and the drain electrode 12 separate from each other and face each other at the upper portion of the active layer 36. Accordingly, the ohmic contact layer 34 on the upper portion of the active layer 36 is exposed, and the exposed ohmic contact layer 34 is removed in patterning of the source electrode 8 and the drain electrode 12.
When the portion of the ohmic contact layer 34 is removed, a portion of the semiconductor layer 32 is exposed, and the exposed portion of the semiconductor layer 32 is defined as a channel region of the TFT.
A passivation film 38 made of SiNx is then deposited onto the gate insulating layer 30 on which the source electrode 8 and the drain electrode 12, etc. are formed with the exposed semiconductor layer 32. This can be done using a CVD (chemical vapor deposition) method. Herein, inorganic substance such as SiNx, etc. is used as a material of the passivation film 38. Recently, in order to improve the aperture ratio of the liquid crystal cell, organic substances having low dielectric constant such as BCB (benzocyclobutene), SOG (spin on glass) or acryl, etc. are used.
A drain contact hole 16 for exposing a part of the drain electrode 12 is then formed by selectively etching the passivation film 38 formed on the drain electrode 12 through a fourth mask.
Thereafter, a pixel electrode 14 is formed by sputtering a transparent electrode substance onto the passivation film 38 and patterning it through a fifth mask. Afterward, patterning is performed so as to make the pixel electrode 14 in contact with the drain electrode 12 through the drain contact hole 16.
Lastly, after forming an oriented film 51 on the TFT, a rubbing process is performed. A first polarizing plate 52 is also formed onto the TFT array substrate 50 opposite to the oriented film 51. Accordingly, the fabrication of the TFT array substrate 50 having the TFT is finished. Herein, the rubbing process means rubbing the surface of the oriented film 51 with a fabric at a uniform pressure and speed so as to arrange polymer chain on the surface of the oriented film 51 in a certain direction in order to determine an early oriented direction.
The fabrication process of the storage capacitor region will now be described in detail with reference to accompanying FIG. 2.
First, the gate insulating layer 30 is formed after patterning the gate line 4 on the TFT array substrate 50. The storage electrode 20 is then patterned on the gate insulating layer 30. Herein, the storage electrode 20 is formed during the patterning of the source electrode 8 and the drain electrode 12 of the TFT. The storage electrode 20 overlaps with a part of the gate line 4 with the gate insulating layer 30 formed between them and is operated as the storage capacitor 18.
The passivation film 38 is then formed on the gate insulating layer 30 on which the storage electrode 20 is formed, and the storage contact hole 22 for exposing a part of the storage electrode 20 is formed by etching a part of the passivation film 38. Herein, the storage contact hole 22 is formed as the drain contact hole 16 of the TFT is formed.
The pixel electrode 14 is then patterned on the passivation film 38, and the pixel electrode 14 is in contact with the storage electrode 20 through the storage contact hole 22. Herein, the pixel electrode 14 is formed in the patterning process of the pixel electrode 14 in the TFT region.
The fabrication process of the color filter substrate 60 having the color filter structure will now be described in detail with reference to FIG. 2.
First, a black matrix 62 is coated onto the color filter substrate 60 (e.g., glass substrate) at regular intervals. A red•green•blue color filter 63 is then formed onto the color filter substrate 60 on which the black matrix 62 is not coated. However, the color filter 63 is extended to certain regions of the black matrix 62.
A common electrode 64 is then formed by forming a metal substance onto the entire upper surface of the color filter 63 including the black matrix 62 and by patterning it. An oriented film 65 is then formed on the entire upper surface of the obtained body, and rubbing is performed. A second polarizing plate 66 is also formed on the opposite surface of the obtained body (color filter substrate 60) so as to correspond to the oriented film 65. Accordingly, the fabrication of the color filter substrate 60 having the color filter structure is finished.
When the fabrication of the TFT array region and the color filter region is finished as discussed above, a sealant (not shown) is printed onto the TFT array substrate 50, and the spacer 70 is dispersed on the color filter substrate 60. Herein, according to circumstances, the spacer 70 is dispersed on the TFT array substrate 50, and the sealant is printed onto the color filter substrate 60.
Afterward, the TFT array substrate 50 and the color filter substrate 60 having the above-described structures are adhered to each other. The adhered TFT array substrate 50 and the color filter substrate 60 are then cut into unit liquid crystal display panels. Herein, because plural liquid crystal display panels are simultaneously formed onto a glass substrate having a large area to improve a yield rate, a cutting process is required.
A liquid crystal layer 80 is formed at a space between the oriented film 51 of the TFT array substrate 50 and the oriented film 65 of the color filter substrate 60 by injecting liquid crystal into the cut liquid crystal display panel through an injecting hole. Thereafter, the injecting hole is sealed. Herein, in an early liquid crystal display fabrication process, after injecting the liquid crystal into the plural liquid crystal display panels, the liquid crystal display panels are cut into unit liquid crystal display panels. However, if the size of the unit liquid crystal display panel increases, uniform liquid crystal injection become more intricate and productivity is lowered due to liquid crystal injection errors. To address this problem, a method which injects liquid crystal after cutting is preferred.
Because the unit liquid crystal display panel has minute cell-gap having a μm size in several hundred mm2 area, in order to inject the liquid crystal efficiently, a vacuum injection method using a pressure difference between the inside and outside of the unit liquid crystal display panel is generally used.
The light transmittance process of the liquid crystal display panel fabricated by the above-described process will now be described in more detail with reference to accompanying FIGS. 1 and 2.
First, a common electrode voltage is supplied to the common electrode 64 formed on the surface of the color filter substrate 60 as one body. And a scanning signal is sequentially supplied from the gate driver IC (not shown) formed on the TFT transistor array substrate 50 to the gate line 4. Accordingly, the liquid crystal cells arranged in the matrix form are sequentially selected by the gate line units.
The scanning signal supplied to the liquid crystal cells of the selected gate line 4 is applied to the gate electrode 10 of the TFT of each cell, and accordingly a conductive channel is formed between the source electrode 8 and the drain electrode 12.
In the meantime, a data signal is supplied from the data driver IC (not shown) to the liquid crystal cell of the selected gate line 4 through the selected data line 2, and the supplied data signal is applied to the source electrode 8 of the TFT. Accordingly, the data signal applied to the source electrode 8 of the TFT is supplied to the drain electrode 12 through the conductive channel for a scanning signal apply period. The data signal supplied to the drain electrode 12 of the TFT is supplied to the pixel electrode 14 in contact with the drain electrode 12 and operates the liquid crystal with the common electrode voltage supplied to the common electrode 64 of the color filter substrate 60.
Because the pixel electrode 14 contacts the storage electrode 20 through the storage contact hole 22, the data signal supplied to the pixel electrode 14 is supplied to the storage electrode 20 for the scanning signal apply period and is charged to the storage capacitor 18. The voltage charged in the storage capacitor 18 is then supplied to the pixel electrode 14 for the TFT turn-off period in which the scanning signal is not applied, and accordingly the operation of the liquid crystal is maintained.
As described above, by applying the common electrode voltage to the common electrode 64 formed on the surface of the color filter substrate 60 as one body and applying the voltage of the data signal to the pixel electrode 14 of the liquid crystal cells on the TFT array substrate 50 selected by the gate line units, the electric field is applied to the liquid crystal layer 80 between the common electrode 64 and the pixel electrode 14.
When the electric field is applied to the liquid crystal layer 80, the liquid crystal is rotated by dielectric anisotropy and transmits the light generated by the back light unit from the TFT array substrate 50 to the color filter substrate 60 through the pixel electrode 14, the liquid crystal layer 80 and the common electrode 64. Herein, according to an amplitude of the voltage of the data signal applied to the pixel electrode 14, strength/weakness of the electric field is adjusted, and the light transmittance of the liquid crystal layer 80 is adjusted by the strength/weakness of the electric field.
When the electric field in a certain direction is continually applied to the liquid crystal layer 80, however, the liquid crystal is deteriorated. Accordingly, in order to prevent the deterioration of the liquid crystal, the data signal voltage value is repeated as positive/negative as it is applied to the common electrode voltage. This is called a reverse operating method.
As described above, the liquid crystal display displays pictures by adjusting light transmittance, and the picture quality is affected by the light transmittance. Thus, a product having a good light transmittance can display bright and clear pictures.
FIG. 3 is an exemplary view illustrating the light transmittance of a general liquid crystal display panel.
FIG. 3 illustrates the TFT array substrate 50 and the color filter substrate 60 facing to each other; the liquid crystal layer 80 filled between the TFT array substrate 50 and the color filter substrate 60; and the wirings 53 and the black matrix 62 respectively patterned on the TFT array substrate 50 and the color filter substrate 60 so as to correspond to each other.
Accordingly, the light emitted from the back light unit (not shown) is transmitted through the pixel region of the liquid crystal display panel and displays a picture. However, because the light emitted from the back light unit is reflected at the region on the TFT array substrate 50 in which the wirings 53 are formed, light transmittance of the liquid crystal display panel is lowered. Thus, in the conventional liquid crystal display panel, light transmittance is not good. In order to solve this problem, a liquid crystal display panel for improving the light transmittance has been presented.
FIG. 4 is an exemplary view illustrating the light transmittance of a the liquid crystal display panel having improved light transmittance. In comparison with the conventional liquid crystal display panel in FIG. 3, it further includes a plurality of micro lenses 90 corresponding to gaps between the wirings.
The micro lens 90 is fabricated as a unit pixel shape, is aligned with gaps between the wirings 53 and is adhered to the TFT array substrate 50 so as not to face the color filter substrate 60. In more detail, the micro lenses 90 are adhered to the surface not having the wirings 53 and facing the back light unit.
The micro lens 90 refracts the light transmitted from the back light unit to the TFT array substrate 50 toward the pixel region of the liquid crystal display panel in order to improve a flux of light being transmitted through the pixel region of the liquid crystal display panel. However, in the fabrication of such a liquid crystal display panel including the micro lenses 90 between the wirings 53, by additionally fabricating the micro lenses 90 in a unit pixel shape, aligning each micro lens 90 and adhering it with the unit pixel of the liquid crystal display panel, the productivity of the liquid crystal display panel may be lowered, and accordingly a production cost may be increased.
In addition, because the micro lens 90 adhered to each unit pixel of the liquid crystal display panel increases the weight and thickness of the liquid crystal display panel, it is difficult to lower the weight of the liquid crystal display and slim down the liquid crystal display.
Further, because the light generated by the back light unit is transmitted through the micro lenses 90 in the pixel region of the liquid crystal display panel, a refractive index difference arises due to the different materials of the TFT array substrate 50 and the micro lenses 90, and as a result, light reflecting elements are generated. Accordingly, the light transmittance of the liquid crystal display is not remarkably improved in comparison with improvement of light usage efficiency.